Conceptual design of non-destructive time profile monitor for femtosecond long electron bunches I. V. Konoplev, A. J. Lancaster, H. Harrison, F. Bakkali Taheri, G. Doucas. John Adams Institute for Accelerator Science, Department of Physics, University of Oxford Denys Wilkinson Building, Keble Road, Oxford, OX1 3RH Ivan.konoplev @physics.ox.ac.uk

Overview Introduction Historical overview: E203 experiment and its limitations Experiments at LUCX, KEK Conceptual design of single-shot monitor Conclusion

Motivation Demand for non-destructive, single shot, accurate, compact and cost effective diagnostics Many applications: NGLSs, FELs, wakefield driven accelerators; require or generate short fs long electron bunches or modulated beams with fs periodicity. Understanding of a bunch profile improves understanding of: wakefield plasma interaction and radiation generation; beam – beam interactions ….

Coherent radiation spectroscopy for beam profile measurements Coherent Synchrotron radiation Free Electron Lasers Coherent Smith-Purcell radiation (cSPr) Coherent Diffraction and Transition radiation

Smith-Purcell Radiation Coherent, polarised radiation emitted from a periodic structure Induced by a charged bunch travelling above the structure Radiation has angular dispersion – frequency spectrum can be measured Longitudinal bunch profile is encoded within the frequency spectrum

Experimental set up E203 at FACET, SLAC E203 set-up consisted of vacuum chamber, (a) carousel with 3 gratings and a flat piece of metal (‘blank’). Outside the chamber: (b) system of changeable filters, (c) Winston cones and 11 pyroelectric detectors. Gratings and filters are interchanged during operation. (a) (b) ≈20 GeV electrons 0.5 - 2.0 x 1010 electrons per bunch Normalized emittance 60 mm-mrad Bunches at 10 Hz (c) Fined synonym for remotely, arranged along beam line

E203 Schematic layout Θ=140 Remove it Θ=40

fs-Time bunch profile SP-monitor Current average profile bunch monitor (20GeV, sub-ps electron bunches at SLAC) Carousel with 3 gratings and blank Normalised amplitude Green markers - 1.5 mm grating Blue markers – 0.25mm grating Red markers - 0.05 mm grating KK technique References need to be moved to separate slide Frequency (THz) The most recent experiments were to study the properties of cSPr: - polarization - directionality

Phase reconstruction software Schematic Diagram of PCI Algorithm

Phase reconstruction software Examples of pulse reconstructions Pulse reconstruction using PCI algorithm (red line) and iterative algorithm (blue line). Original pulse is dotted black line Normalised amplitude Pulse reconstruction using experimental data observed at FACET SLAC (E203 experiment) Move to FACET PCI algorithm - red line; Iterative algorithm - green line; KK algorithm - blue line Current (arb. units)

Limitations of the E203 monitor Multi-shot measurements Requires minimum 4 sets of measurements: Three different gratings on a carousel One “blank” measurement Mechanically complex: Carousel rotation Carousel translation Changing filters Components must be λ-independent Solutions: Use polarization of cSPr for background separation Use directionality of cSPr to avoid cross-talk

1/ Coherent SP radiation studies at LUCX, KEK to confirm proposed solutions 2/ Conceptual design of single shot cSPr monitor

Numerical predictions of cSPr at LUCX ≈8 MeV electrons 50 pC or 1.0 x 108 electrons per bunch Normalized emittance ~100 mm-mrad Number of micro-bunches per train 4 Repetition rate 12Hz

LUCX cSPr experimental layout Electron beam To cover the same frequency range at LUCX as it was at FACET 10MeV beam should be 0.1mm away to generate radiation at 10THz LUCX vacuum chamber with gratings installed Vacuum chamber Photo will be sent with appropriate grating remove eqs add bullet points about measurements Beam splitter Fixed mirror Parabolic mirror moving mirror VDI ZB detector

Interferometric frequency measurements cTr measured to study the detector response cSPr measured at 90 degree observed from 1mm grating cSPr measured at 90 degree observed from 1mm grating The detector frequency range from 290GHz to 420GHz

Preliminary polarisation study Detector and polariser are rotating together Rotatable polariser cSPr after interferometer Rotatable detector Preliminary estimation of cSPr degree of polarisation: cSPr ~ 0.9

Degree of polarisation of background radiation Preliminary estimation of background radiation degree of polarisation: ~ 0.14

Preliminary conclusion We can measure cSPr generated by LUCX beam (very different from FACET beam) The cSPr degree of polarisation agrees with predicted values and can be distinguished from background

Conceptual design of a single shot, non-destructive fs-bunch monitor For the next generation of linear colliders, wake-field accelerators and light sources (X-ray and THz FELs) to understand/control the physics phenomena and machine operation. 1/ Truly single shot monitor (1 bunch = 1 profile) 2/ Compact 3/ Applicable to all fs-particle accelerators including plasma assisted wake-field accelerators

Schematic Grating Layout

Multi-grating layout Background radiation is subtracted via polarisation measurements and analysis Gratings will be longitudinally spaced to avoid geometry problems Rotated around azimuthal angle to reduce length of the apparatus

Femtosecond resolved imaging of e-bunch longitudinal profile Detector   mirror polariser Winston cones Incoming signal Vacuum chamber 11 output ports Beam path Single grating Background output window and absorbing material output port Consistency with other overheads

Femtosecond resolved imaging of e-bunch longitudinal profile 11 output ports If funds are granted the monitor will be ready in 2017 and installed on CLARA beam line and tested in 2018 Beam path 11 output ports Consistency with other overheads 11 output ports 1/ Polarisation and frequency resolved - in the approximate 64cm beam-line footprint system 2/ Spectral coverage from 0.1THz up to 15THz 3/ Single shot, non destructive, fast beam profile monitor

Conclusion Overview of cSPr bunch profile monitor (E203 experiment at FACET, SLAC) Phase reconstruction algorithm Limitations of the original monitor Conceptual design of a single shot monitor Studies of cSPr radiation Next step (if funds will be available) is to construct and test the monitor at CLARA (UK) facility by the end 2018.

Thank you Acknowledgement: STFC PRD grant (STFC PRD, ST/M003590/1); Leverhulme Trust Network Grant (IN-2015-012)

References 1/ F. Bakkali Taheri, I. V. Konoplev, et al., Phys. Rev. Accel. And Beams, 19, 032801, 2016. 2/ H. Harrison, et al., I. Konoplev, et al., 6th IPAC 2016: Proceedings, 7-13 May, 2016. 3/ F. Bakkali Taheri, I. V. Konoplev, et al., 6th IPAC 2016: Proceedings , 7-13 May, 2016. 4/ H.L. Andrews, et al., Konoplev, I.V., et al., Phys. Rev. Special Topics - Accelerators and Beams, 17 (5), 2014. 5/ H.L. Andrews, et al., I.V. Konoplev, et al., Nucl. Instrum. Meth. in Phys. Res., A, 740, pp. 212-215 (2014). 6/ N. Fuster-Martínez, et al., I.V. Konoplev et al., 4th IPAC 2013 Proceedings, pp. 634-636, 2013. 7/ F.B. Taheri, I.V. Konoplev, et al., 4th IPAC 2013 Proceedings, pp. 801-803, 2013. 8/ I.V. Konoplev, et al., International Conference Proceedings, IRMMW-THz, 6665429, 2013.